Coating method for enhancing punchability of steel sheet

ABSTRACT

A COATING ON THE SURFACE OF STEEL SHEET SUCH AS ELECTRICAL STEEL SHEET INCLUDING SILICON STEEL AND LOW CARBON IRON TO IMPROVE THE PUNCHABILITY OF THE STEEL SHEET, SAID COATING COMPRISES CHROMIC ACID, CALCIUM AND AN ORGANIC POLYHYDRIC ALCOHOL.

y 5, 1971 HIROSHI SHIMANAKA ETAL 3,591,425

COATING METHOD FOR ENHANCING PUNCHABILITY OF STEEL SHEET Filed Oct. 9, 1968 2 Sheets-Sheet 1 Fig. I

4 umqmma 0 000000 Number of Punchings unfll Burr Height Reaches 50, -lm n6 :2 I6 20 24 2e (00 (00 6o (00 I00 Rojio 9f Colciqm to Chromic g 2 Acld |n Solunon I so 40- I: 31 (A) I 0 I I I Y I 1 2O 4O 6O 80 I00 I20 I40 Number of Punchings (H0 y 6, 1971 HIROSHI SHIMANAKA ETA-L 3,591,425

COATING METHOD FOR ENHANCING PUNCHABILI'IY OF STEEL SHEET Filed Oct. 9, 1968 2 Sheets-Sheet 2 Fig. 3c]

Cooting(A)of the Presentlnvention (magnification 100 x) At the Beginning of Punching After I30 X IO Times of Punching Fig. 3b

Phosphate Coating (C) (magnification I00 At the Beginning of Punching After l2 X I0 Times of Punching United States Patent 3,591,425 COATING METHOD FOR ENHANCING PUNCHABILITY OF STEEL SHEET Hiroshi Shimanaka, Toshio llrie, and Toshikuni Tanda,

Ashiya-shi, Japan, assignors to Kawasaki Steel Corporation, Fukiai-ku, Kobeshi, Japan Filed Oct. 9, 1968, Ser. No. 766,266 Claims priority, application Japan, Oct. 18, 1967, 42/ 67,027 Int. Cl. C23f 7/26 US. Cl. 148-62 7 Claims ABSTRACT OF THE DISCLOSURE A coating on the surface of steel sheet such as electrical steel sheet including silicon steel and low carbon iron to improve the punchability of the steel sheet, said coating comprises chromic acid, calcium and an organic polyhydric alcohol.

The present invention relates to coating on a surface of steel sheet such as electrical steel sheet including silicon steel and low carbon iron, thereby to improve punchability or punching quality of the steel sheet.

Whenever an electrical steel sheet or a silicon steel sheet is used to manufacture a motor, generator, small size transformer or the like, it is usually punched into a desired shape. The punchability of the steel sheet is, therefore, a very important problem in core manufacturing, and much attempts have been done to improve the punchability. The improvement of punchability means the increase of the number of punchings per grinding of die, that is, to lengthen a life-time of the die.

The die life depends upon not only die material, clearance, punching oil or other factors on working side but also mechanical properties or surface coating of the material to be punched. If the burr height of the punched material increases, when laminated, each adjacent sheet will be electrically short-circuited to damage electro-magnetic properties and to cause trouble in handling. Hence, when the burr height exceeds a predetermined value, the punching operation is interrupted to take off the die to grind. The maximum value of the burr height is usually 50-120,, and in case of steel die, it may be used as much as fifty thousand to a hundred thousand punchings before its burr height reaches said value. Recently, by use of a super hard alloy such as sintered WC-Co alloys as die material, the die life has been lengthen to five hundred thousand to a million times, which are about ten times to the die life of a steel die. But the sintered WC-Co alloys include many disadvantages; they are very expensive and also require much difficulty in fabricating and grinding of the die.

As to the material to be punched, when a grain oriented silicon steel is to be used, an insulating coating of MgO-SiO type damages the die so that it has been proposed to suppress the formation of such coating or to remove it by after-treatment such as pickling. For a nonoriented electrical sheet, it has been found that an enamel or varnish coating improves the punchability and it is commercially available. Even when the enamel or varnish coating is used, the die life is at most two hundred thousand to four hundred thousand punchings for a steel die. Further, such organic coating has a serious disadvantage that it is unendurable to a high temperature heat treatment such as stress relief annealing at about 800 (3., because of its poor heat resistance at about 300 C.

Some inorganic coatings increase the die life 20-30% while any inorganic coating has not been known which improves the die life three to five times as in the case of organic coating.

The present invention can remarkably lengthen the die life to one and a half million times or more for a steel die by applying a thin, uniform inorganic coating on a surface of a steel sheet, without changing any mechanical and magnetic properties. This increased value of the life are astonishing figures amounting to l0-20 times as compared to the conventional inorganic coating, and 2-5 times as compared to the organic coating such as enamel or varnish. Further, the coating of the present invention is applicable to an electrical steel sheet including less than 4% of Si and this coating provides excellent properties required for an insulating coating for electrical steel sheet, such as interlamination insulating resistance, space factor, corrosion resistance, adhesion, heat resistance, weldability and so on.

The essentials of the present invention is to improve the punchability of the steel sheet by forming a coating on the surface of the sheet; that is to say, by the steps of applying a solution containing, by weight ratio, parts of chro mic acid, 16-20 parts of calcium and 820 parts of organic polyhydric alcohol on a cleaned surface of a steel sheet, and then baking said steel sheet. The term cleaned surface means a surface free from oil and dust and does not mean to eliminate a thin surface layer derived during a steel plate manufacturing process, such as an oxide film formed during a continuous open annealing.

Chromate such as zinc bichromate, magnesium chromate have been hitherto utilized as agents for insulating coating of electrical steel sheets.

It has also been proposed to add divalent metal oxide to chromic acid and mix them with a reducing agent and metal oxide or its double salt. However, no information has been given to the improvement of the punchability by the use of particular proportion of calcium to chromic acid as in the present invention.

The method of the present invention will now be described more particularly.

The solution containing, by weight ratio 20 parts of calcium to 100 parts of chromic acid corresponds to an aqueous solution of calcium bichromate and when calcium is contained below 20 parts the solution corresponds to an aqueous solution of both calcium bichromate and chromic acid. However, the solution for the present invention can be obtained not essentially by using calcium bichromate but by using an aqueous solution of chromic acid to which one or more kinds selected from the group consisting of CaO, Ca(OH) CaCrO, and CaCO are added to maintain the ratio of calcium to chromic acid between 16/ 100-20/100. If the solution contains calcium of more than 20 parts against 100 parts of chromic acid, an insoluble precipitate is formed, and when the solution containing such precipitate is applied on a surface of a steel sheet and then baked, the lusterless and non-uniform coating is formed causing poor punchability as a result.

When calcium is less than 16- parts to 100 parts of chromic acid is included, a low hardness coating is formed and so it is easily scratched and has a poor punchabiltiy. Generally, it is preferable that the hardness of coating is more than 3H in the pencil hardness.

The invention can be more fully understood from the following detailed description when taken in connection with the accompanying drawings, in which:

FIG. 1 is a graph showing the relation between the ratio of calcium ions and chromic acid and the punchability of the coating;

FIG. 2 comparatively shows the punchability of the steel sheets with the coating in accordance with one embodiment of the present invention and with those of a conventional coating;

FIG. 3a is a (photographic) representation showing a cross-sectional view of a punched portion of a steel sheet with the coating (A) in accordance with an embodiment of the present invention; and

FIG. 3b is a (photographic) representation showing a cross-sectional view of a punched portion of a steel sheet with a conventional phosphate coating (C).

Now, referring to the drawings, FIG. 1 shows the relationship between the ratio of calcium to chromic acid and the punchability of the coating thus formed, in which a solution having 12 parts of ethylene glycol added to 100 parts of chromic acid as a reducing agent is applied onto an electrical steel sheet having 0.3% Si, and baked. The testing procedure of the punchability is carried out by punching disc of 20 mm from said coated sheet by means of a steel die until the burr height reaches 50,41"

As seen in FIG. 1, a good punchability will be presented when the ratio of Ca/CrO is Within 16/ 100-20/ 100. Eurther, it is preferable that the ratio of calcium in this connection is within 18.5-19.7 since about 100 thousand punchings are obtained.

As an organic polyhydric alcohol which serves as a reducing agent for chromic acid may be applicable to glycerine, sucrose, or grape sugar instead of ethylene glycol, but ethylene glycol is preferable because of having good wettability and uniformity. For example, if less than 8 parts of ethylene glycol is included in 100 parts of chromic acid, reduction will not be completed without a high temperature, long time baking, and the coating tends to be hygroscopic in air.

If the baking is so controlled as not to become hygroscopic, it results in a poor adhesion of the coating which will be easily removed by a subsequent working and thus does not improve the punchability. On the other hand, when more than 20 parts of ethylene glycol is added, reduction of a solution itself will take place in a short time to produce a precipitate, and hence such solution is not available. Moreover, an optimum range of baking condition is so narrow that a coating of poor adhesion will be produced, which will be removed during punching and dust accumulates in the die to cause trouble in handling.

Besides the above compositions, a small amount of non-ion type surface active agent may be added to the solution in the present invention to improve wettability and uniformity to a steel sheet without damaging punchability and other properties of the coating formed. When Actinol (a kind of polyoxyethylene alkyl ether) is used, the amount of 0.0020.5% will provide good wettability and improve uniformity of the coating.

The relationship between the baking temperature and baking time cannot be strictly defined because it varies with sheet thickness, sheet size and heat capacity of furnace. For example, when a sheet of 0.5 mm. thickness is baked using a sufliciently large heat capacity furnace for that sheet, if the baking temperature is less than 200 C., it takes more than minutes of baking time to get a coating having an adequate hardness, good adhesion and nonhygroscopic. Such a baking temperature is not preferable.

When baking temperature exceeds 500 C., an optimum range of baking time becomes so narrow that hygroscopicity due to shortage of baking time and poor adhesion due to overbaking under high temperature will occur. Thus, in the present invention, a suitable coating may be produced with a proper selection of baking time within the baking temperature of 200500 C., and it has been proved by experiments that the preferable temperature is 280-350 C.

When the thickness of the coating in accordance with the present invention is less than 0.5 it will not contribute to an enhancement of punchability. Conversely, the coating of more than 2.5 4 thickness is so soft as to be less than 3H. Such coating is apt to accumulate dust in the die, causing troubles in punching operation. Accordingly, the preferable thickness of coating is within 0.52.5,u, particularly 0.7-1.5,u.

One example of the present invention will now be com- 4 pared with a steel sheet having conventional coatings with respect to their punchability.

(l) STEEL STRIP (SHEET) TO BE PROCESSED A steel strip (C, 0.02%, Si, 0.34%, Mn, 0.30%, P, 0.01%) of 0.5 mm. thickness and Hv=130 is sampled and its surface is cleaned previously.

(2) KINDS OF COATING AND TREATMENT CONDITIONS (A) Coating according to the process of the present invention A solution comprising 120 1. of 18% calcium bichromate solvent, to which 0.5 kg. of chromic acid and 2.5 kg. of ethylene glycol are added, was applied uniformly to the above-mentioned steel strip by means of grooved rubber roller and baked in a furnace at 300 C. for 100 seconds.

(B) Magnesium chromate coating A solution comprising 100 l. of Water into which 70 kg. magnesium chromate is dissolved and to which 5 kg. of ethylene glycol is added, was uniformly applied to the steel strip, and it was baked at 500 C. for 40 seconds.

A phenol alkyd varnish was uniformly applied onto the steel strip and it was baked at 400 C. for seconds.

The coating mentioned in the above items (B), (C) and (D) are conventionally used.

(3) THICKNESS AND HARDNESS OF COATINGS Coating Pencil thickhard- Type of coatings ness t) mass (A) Coating of present invention 1. 2 SE (B) Magnesium chromate coating 1. 0 4H (C) Phosphate coating 0. 6 411 (D) Varnish coating 3.5 4H

1 Average of both planes.

(4) TESTING PROCEDURE OF PUNCHABILITY Discs of 20 mm were punched by the use of a steel die (AISI D-3) with a clearance of 40,14 and punching oil #6300 (made by Nihon Kosakuyu Co.). Three pieces were sampled for every ten thousand punchings and burr height was measured with a dial gauge at 4 points per piece, and the mean value of each maximum value was indicated respectively.

FIG. 2 shows punchability of steel sheets to which the abovementioned coating (A), (B), (C) or (D) is applied, respectively. It is usually known that a commercially available inorganic coating presents 70-100n of burr height after one hundred thousand punchings. The coating (C) used above has substantially same properties as those of commercially available coatings, and thus may be considered as a typical example of commercially available Inorganic coatings.

The steel strip to which coating in accordance with the present invention was applied requires one million and three hundred thousand punchings before the burr height reaches 30p. and this offers a remarkable advantage in that it lengthens the die life as much as 10-20 times in comparison with the commercially available inorganic coatings.

It has been well known that die clearance (gap between upper die and lower die) is one of significant factors which govern punchability, and that a proper value of the clearance varies with sheet thickness, sheet hardness, shape of die and etc. In a complex shaped die comprising a number of punch pieces such as slot of motor core, the clearances at each portion cannot always be kept at a constant value during punching. The steel sheet of 0.5 mm. thickness on which the coating was applied in accordance with the present invention was tested with steel die of different clearance for die abrasion and burr height after a number of punchings. The results were that they were substantially constant when clearance was 30-80,. This means that the coating of the present invention has a good adaptability to various kinds of dies. In eifect, good results in punching test of motor cores were obtained. Normally, the die can be used until the bearing portion (parallel portion) of lower die is worn off. When the steel sheet with the coating of the present invention is punched, since it has large tolerance for clearance, the die can be used beyond the bearing portion worn 0H and thus the total lifetime of die is extremely lengthened. As for punching oil, various kinds of oils varying from low viscosity, volatile ones to high viscosity, non-volatile ones were tested revealing good results regardless of the sort of oils.

FIGS. 3a and 3b each show cross-sectional views of the steel sheets after punched, having the coating (A) of the present invention and the conventional phosphate coating (C). Specimens were taken at the beginning and after one million and three hundred thousand punchings for the coating (A), and at the beginning and after one hundred and twenty thousand punchings, for the coating (C). The sampled pieces are shown by photograph magnified a factor of 100 in cross section.

Referring to FIG. 3(b) in the coating (C) plastic deformation and burr height are small and rectangular shape at the beginning of the punching while exhibits very large plastic deformation and burr height after one hundred and twenty punchings. Whereas the coating (A) of the present invention, as shown in FIG. 3a, exhibits large plastic deformation but small burr height even after one million and three hundred thousand punchings.

The coating of the present invention assumes a lustrous reddish brown color in appearance and provides excellent properties when used for insulating coating for electrical steel sheet. In addition, lowering of the space factor scarcely is recognized because the coating is thin and finished with smooth surface. The interlamination electrical resistance of the coating, according to 11$ Article 2, is proved 2O50SZ-cm. /layer in case the coating is approximately 11.0 in thickness, assuring rather superior properties than the conventional insulating coatings.

TIG welding which was applied to a laminated end surface brought about a fine bead free from blow-hole.

Even when the stress relief annealing may have been carried on in a nitrogen atmosphere at the temperature of 800 C. during 5 hours, there was not much of changing in the good adhesion than before such annealing done, and also the deterioration of the interlamination resistance was slightly observed. Further, the corrosion test in Freon 22 at the temperature of 40 C. during 1700 hours was carried on, and, as a result, no change in properties of the coating in appearance and extremely little change in the weight were proved, which were almost the same as a pure inorganic coating (C).

What is claimed is:

1. In the method of shaping steel sheet wherein said sheet is shaped by die punching, the improvement comprising applying to said sheet an aqueous solution of, in weight ratio, 100 parts of chromic acid, 16-20 parts of calcium and 8-20 parts of an organic polyhydric alcohol, baking said steel sheet to form a coating of about 0.5 to 2.5 in thickness, and shaping said sheet by die punching.

2. The method according to claim 1 wherein said steel sheet applied said solution is baked at the temperature of 200-500 C.

3. The method according to claim 2, wherein said baking temperature is 280-350" C.

4. The method of claim 1 wherein said calcium is from about 18.5 to 19.7 parts by weight.

5. The method of claim 1 wherein said organic polyhydric alcohol is ethylene glycol.

6. The method of claim 1 wherein said aqueous solution contains a surface active agent.

7. The method of claim 1 wherein said coating has a thickness of about 0.7 to 1.5.

References Cit-ed UNITED STATES PATENTS 3,346,522 10/1967 Schuster et al 1486.l6X

RALPH S. KENDALL, Primary Examiner C. WESTON, Assistant Examiner US. Cl. X.R.. 72-47 

